1. The Field of the Invention
The present invention relates to a process for recovering bitumen from tar sands. More particularly, the present invention relates to a modified hot water process suitable for recovering bitumen from certain domestic tar sands.
2. The Prior Art
Tar sands (sometimes also referred to as oil sands or bituminous sands) refers to naturally occurring mixtures of bitumen and sand. Anciently, many civilizations used tar sands (then called "pitch") for waterproofing in the construction of boats and as a cement for building materials. Tar sands were also used as a paving material for road construction in Europe during the Napoleonic era. In the late nineteenth century, the Canadian government began to look to the great reserves located in Canada with the hope of exploiting the tar sands as an energy source.
Tar sands, which are typically dark brown to black in color depending upon the bitumen content and composition, can be described either as sand grains cemented by bitumen or as sandstone impregnated with bitumen. It has been found that regardless of the bitumen content, the sand particles are almost always completely covered by a coating of bitumen. For very low grade tar sands, the thickness of the bitumen layer may be only 0.01 mm or less; in high grade tar sands the bitumen layer may be many times thicker.
The bitumen of tar sand consists of a mixture of a variety of hydrocarbons and heterocyclic compounds. After the bitumen has been separated from the sand, it can be further treated to form a synthetic crude oil suitable for use as a feedstock for the production of gasoline, heating oil, and/or a variety of petrochemicals. The sand component of tar sand is mostly quartz, with minor amounts of other minerals.
Tar sand deposits often occur in the same geographical area as conventional petroleum deposits; tar sand deposits have been found throughout the world, with the exception of Australia and Antarctica. The major known deposits of tar sands are located in Canada, Venezuela, Utah, Europe, and Africa. It is estimated that the Canadian deposit, known as the "Athabasca tar sands", contains nine hundred (900) billion barrels of oil. About sixty-five percent (65%) of all known oil in the world is contained in tar sand deposits or in heavy oil deposits. The Venezuelan deposit of tar sands is estimated to contain approximately seven hundred (700) billion barrels. The United States has twenty-eight (28) billion barrels in its tar sand deposits. Europe has three (3) billion barrels, and Africa has two (2) billion barrels.
Approximately ninety percent (90%) of the known deposits in the United States are located in Utah, with other major deposits being found in California, Kentucky, and New Mexico. Although the twenty-five (25) billion barrels of bitumen located in Utah may seem small in comparison to the Canadian and Venezuelan deposits, Utah tar sands represent a significant energy resource when compared to crude oil reserves in the United States, which are estimated to be approximately thirty-one (31) billion barrels.
Currently, only the Athabasca tar sands located in Canada are undergoing significant commercial exploitation. Nevertheless, current and impending Canadian production only amounts to about sixty (60) million barrels of synthetic crude oil per year.
The tar sands located in the Athabasca deposit differ considerably from those deposits located in Utah and other areas of the world. Analyses of the Athabasca tar sands indicate that the average bitumen content is approximately twelve to thirteen percent (12-13%) by weight. The bitumen content of the Utah tar sands, on the other hand, varies from about five percent (5%) to about thirteen percent (13%) by weight, with the average of all deposits being slightly less than ten percent (10%) bitumen by weight.
Additionally, Athabasca tar sands have a relatively high moisture content of approximately three to five percent (3-5%) by weight water. This water, which is referred to as connate water, forms a thin film between the bitumen and the grains of sand. On the other hand, the Utah tar sands are so dry that their moisture content cannot be detected by standard analytical methods.
More significantly, the bitumen viscosity of the Utah tar sands is approximately one or two orders of magnitude greater than the bitumen viscosity of the Athabasca tar sand. FIG. 1 illustrates the average bitumen viscosities of various Utah tar sand deposits as compared to the Athabasca tar sand deposit; note that viscosity is plotted in an exponential scale.
Another difference is that, unlike Athabasca tar sand, domestic (and particularly Utah) tar sands are usually consolidated. Therefore, they will not undergo significant size reduction by ablation during the digestion step. Accordingly, some preliminary size reduction is required before digestion, such as crushing or grinding. A hot water process for use in processing the Athabasca tar sands has been developed; while this process has undergone many modifications over the years, it has maintained its essential characteristics. In 1944, the fundamental features that characterize this hot water process were set forth in K. A. Clark, "Hot Water Separation of Alberta Bituminous Sand," 47 Canadian Institute of Mining and Metallurgy Trans., 255 (1944). This process today forms the basis of the commercial operations which are now used for processing Athabasca tar sands.
Athabasca tar sands are mined with large bucket wheel excavators and/or drag lines. The tar sand is transported to the processing plant by conveyor belt where it is fed into a rotating conditioning drum and mixed with hot water and steam. Sodium hydroxide is added to control the pH of the mixture and keep it basic. As the tar sand is heated, the larger lumps break up, and the bitumen is displaced from the sand particles. The pulp is then discharged from the conditioning drum at about 85.degree. C. and is screened to remove tramp materials and tar sand lumps.
The pulp is then fed to a gravity settler where the initial phase separation of bitumen from sand occurs. The bitumen floats to the surface of the settler where it is removed by radial arms and most of the sand sinks to the bottom where it is discharged as a tailings product. A middling stream is extracted from the side of the settler and is either recycled to control pulp density or is fed to a standard flotation cell for further processing. The bitumen concentrate produced according to this process contains about eight four percent (84%) bitumen and about sixteen percent (16%) mineral matter on a dry basis. Approximately ninety percent (90%) of the total amount of bitumen is recovered in the concentrate.
Because of the vast amount of information available on the Athabasca process and its proven success in Canada, the initial attempts to process Utah tar sands utilized the same process. Due to differences in tar sand characteristics, specifically viscosity and connate water, this process proved to be ineffective on the Utah tar sands. Attempts were made to modify the process to specifically treat Utah tar sands. One successful process is disclosed in U.S. Pat. No. 4,120,776 issued to Miller et al.
The Miller process basically comprises digesting the tar sands in a mixer where high shear forces can be achieved while controlling the percent solids in the mixture, the pH range as determined by the concentration of a particular wetting agent, and the temperature. In order to maintain a high shear force field in the digester, the percent of solids is preferably within the range of about sixty to eighty percent (60-80%), and, in no case, less than fifty percent (50%). A caustic wetting agent such as sodium hydroxide is added to the digester with a concentration range between 0.2 normal and 1.0 Normal, with the most effective range being between 0.5 Normal and 0.8 Normal to maintain a pH of 10 or more.
The contents in the digester are maintained at a temperature above 70.degree. C. and preferably near the boiling point of the aqueous solution. After phase disengagement of the bitumen from the tar sand takes place in the digester, phase separation is achieved in a separation or flotation cell where additional water is added to lower the solids concentration below about fifty percent (50%) solids. The pH of the flotation cell is maintained above 10, and air is diffused into the cell to assist in the phase separation.
An additional process for recovering bitumen from tar sands is disclosed in U.S. Pat. No. 4,174,263 issued to Veatch et al. Basically, this process comprises treating the tar sands with a small amount of liquid hydrocarbons or halogenated hydrocarbons which are capable of penetrating the bitumen, vaporizing at least some of the liquid which has been absorbed into the tar sand in such a manner that the density of the bitumen is reduced, and separating the bitumen from the remainder of the tar sands using a flotation process. The liquid selected to treat the tar sands according to this process must have a relativey low boiling point; otherwise, large amounts of energy are needed to vaporize the liquid in order to reduce the density of the bitumen.
Another type of process which has been developed to separated bitumen from tar sands utilizes massive amounts of solvent to completely dissolve the bitumen from the tar sands. This process is extremely expensive, because it requires large amounts of solvents which must be recovered and recycled to treat additional tar sand. Also, significant amounts of diluent are lost through evaporation or in the tailings stream. One such solvent extraction process is disclosed in U.S. Pat. No. 4,067,796 issued to Alford et al.
Although many processes have been developed to separate bitumen from tar sand, none of the processes developed to date is effective in processing all of the various types of tar sands located in Utah and other areas of the United States. The hot water process used for the Athabasca tar sands of Canada is ineffective because of the low level of connate water in the Utah tar sands and because of the higher viscosities of the Utah bitumen.
The Miller hot water process U.S. Pat. No. 4,120,776, which was developed especially for Utah tar sands, has proven to be effective on high grade Utah samples such as those from the Asphalt Ridge and P.R. Springs, but it has not been found to be completely successful on the low grade tar sands such as those located in the Sunnyside and Tar Sand Triangle deposits. See J. E. Sepulveda and J. D. Miller, "Extraction of Bitumen From Utah Tar Sands By A Hot Water Digestion-Flotation Technique," 30 Mining Engineering, 1311 (1978).
Additionally, the hot water Veatch processes and the volatilization process (U.S. Pat. No. 4,174,263) all require large amounts of energy to separate the bitumen from the tar sand. Hot water processes operating at 95.degree. C. require at least 45 kilowatt hours of energy per ton of tar sand processed. Much of this energy is wasted in heating the sand particles, which make up the bulk of the material, and which is ultimately discarded as tailings. In addition, the dissolution techniques require large amounts of solvent which becomes very expensive in a large operation because of the significant amount of solvent which is lost in the tailings. Moreover, additional processing of the bitumen is required to recover the solvent for recycling.
In view of the foregoing, it would be a significant advancement in the art to provide an effective process for the separation of bitumen from tar sands which can be readily adapted to process deposits of differing viscosities, and degrees of consolidation. It would be a further advancement in the art to provide such a process which had relatively low energy demands and costs. Such a process is disclosed and claimed herein.